The present invention relates generally to superconducting tape, and more particularly to a metal reinforced superconducting tape.
Synchronous electrical machines having field coil windings include, but are not limited to, rotary generators and rotary motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core and one or more coil windings mounted on the rotor core. The rotor cores may include a magnetically-permeable solid material, such as an iron-core rotor.
Recently, superconducting coil windings have been developed for rotors. Superconducting coils are advantageous because they have effectively no resistance. High temperature superconducting coil windings are formed of brittle ceramic superconducting tape. The tape may be formed of a high temperature superconducting material, such as a Bi1.8 Pb0.4 Sr2 Ca2 Cu3 O10+x (BSCCO 2223), Bi2 Sr2 Ca1 Cu2 O8+x (BSCCO 2212), and YBa2 Cu3 O7xe2x88x92x (YBCO 123). This tape must be cooled to a temperature at or below a critical temperature to achieve and maintain superconductivity.
When this superconducting tape is put under tension, the ceramic grains in the tape degrade, resulting in a decrease in the critical current carrying capability of the superconducting tape. To address this issue, a paper by K. G. Herd et al. titled xe2x80x9cDevelopment and Fabrication of a Bi-2223 Racetrack Coil for Generator Applicationsxe2x80x9d, in IEEE Transactions on Applied Superconductivity, Vol. 7, No. 2 (June 1997) page 531, incorporated herein by reference, describes an epoxy impregnated silver sheathed BSCCO 2223 superconducting tape laminated to copper foils for strength enhancement.
Furthermore, U.S. Pat. No. 5,987,342, incorporated herein by reference, discloses laminating the superconducting tape to a pre-tensioned metal (i.e., steel, copper or superalloy) tape having a thermal expansion coefficient greater than that of the superconducting tape. The metal tape may be laminated to the superconducting tape using solder, epoxy or solderless ultrasonic welding. As the tapes cool after a high temperature lamination, the metal tape causes the superconducting tape to be placed under compression due to the difference in the coefficients in thermal expansion and due to the pre-tensioning of the metal tape. The metal tape improves the superconducting tape""s tolerance to tensile and bending stresses, by placing the superconducting tape under compression.
In accordance with one preferred aspect of the present invention, there is provided a reinforced superconducting tape comprising a superconducting tape comprising a superconducting ceramic material, a first metal reinforcing layer having a greater coefficient of thermal expansion than that of the superconducting tape, and a second metal reinforcing layer having a greater modulus of elasticity than the superconducting tape and the first reinforcing layer.
In accordance with another preferred aspect of the present invention there is provided an electric machine, comprising a stator comprising stator coils, a rotor comprising a rotor core, and a coil winding around the rotor core. The coil winding comprises a superconducting tape comprising one or more ceramic superconductor filaments located in a noble metal sheath, a first metal reinforcing layer having a greater coefficient of thermal expansion than that of the superconducting tape, and a second metal reinforcing layer having a greater modulus of elasticity than the superconducting tape and the first reinforcing layer.
In accordance with another preferred aspect of the present invention, there is provided a method of making a reinforced superconducting tape, comprising providing a superconducting tape comprising a superconducting ceramic material, forming a first metal reinforcing layer having a greater coefficient of thermal expansion than that of the superconducting tape, and forming a second metal reinforcing layer having a greater modulus of elasticity than the superconducting tape and the first reinforcing layer.